Antenna arrays are finding increasing use in various communication systems. A notable use of such arrays is for generating a plurality of beams for satellite-based cellular communications systems, but array antennas are widely used for radar and other sensors. In many cases, circularly-polarized elements are desired for use in an antenna array, for reasons which include polarization independence when receiving signals from linearly-polarized mobile user terminals which may be variously oriented, or for maintaining a particular level of transmitted power while maintaining relatively low electric field strength for each polarization.
Those skilled in the art know that antennas transduce bidirectionally between guided waves and unguided (free-space) waves. Antennas are reciprocal devices, which have the same parameters whether operated in a transmission mode or in a reception mode. More particularly, a transmitted "beam" has the same angular dimensions and gain as a "receive" beam, and the impedance at the guided-wave "feed" is the same in both transmission and reception modes. For historical reasons, the guided-wave terminals are referred to as "feed" terminals, regardless of whether the antenna is operated in a transmit mode or in a receive mode. Ordinarily, the guided-wave "feed" terminal(s) are coupled or connected to a "transmission-line," which as known is a guided transmission path having relatively low transmission loss andor relatively constant characteristic impedance between its terminals, but the feed terminals may be coupled directly to a source or utilization device, such as an electrode of a transistor, without an intermediary transmission line. Those skilled in the art also know that the terms "linear" and "circular" polarization refer to idealized conditions, seldom found in practice. Thus, a "linearly" polarized antenna or antenna element will, in actual use, often display or exhibit some residual response to the cross or orthogonal polarization relative to the nominal principal polarization. Similarly, a "circularly" polarized antenna or antenna element will often exhibit some response to the opposite hand of circular polarization relative to the nominal hand of polarization, and this cross-polarization component may be manifested as a finite "axial ratio." The axial ratio is the ratio, generally specified in decibels (dB), indicating the ratio of maximum to minimum amplitude response to all polarizations. A linearly polarized antenna or antenna element, if it were perfect, would exhibit an infinite axial ratio, while a perfect circularly polarized antenna would exhibit an axial ratio of zero dB. Real antennas, therefore, ordinarily exhibit an axial ratio which is neither zero dB nor infinite dB, and thus the terms used to categorize various types of antennas as circularly or linearly polarized must be understood to inherently include the word "nominally."
Crossed slot antennas are advantageous in that they are lightweight and do not project above the surface or plane of the array. A disadvantage of crossed slot antennas is that they cannot be fed in a simple manner except by a travelling wave in a waveguide context. Copending U.S. patent application Ser. No. 09/496,524, filed Feb. 2, 2000 in the name of Lier, describes an antenna array arrangement operating in disparate frequency bands, which includes an arrangement for feeding crossed slot antennas in one of the bands. The Lier arrangement requires multiple layers of microstrip or stripline transmission lines to effectuate the feed, which may not be advantageous, especially when power levels are significant. A salient disadvantage of slot antennas is that each slot tends to have a gain corresponding to that of an equivalent dipole, which is in the vicinity of 3 dB. Since the gain of an array antenna is dependent, in part, on the gain of the individual antenna elements of the array, an array fitted with slots as the radiating elements must, in order to provide a given gain, be larger in area andor in the number of antenna elements than a corresponding array using antenna elements having gain greater than 3 dB.
U.S. Pat. No. 5,258,771 issued Nov. 2, 1993 in the name of Praba describes an antenna array operating in two disparate frequency bands, which uses interleaved axial-mode "helical" antenna elements. Each such helical antenna includes a helically disposed electrically conductive element with a feed point adjacent a ground plane disposed orthogonal to the axis of the helix. Such helical antenna elements are well known, and have the advantage, when so fed against a ground plane, of providing moderately high gain, together with circular polarization. In order to reduce the interaction between the helical antenna elements of the arrays at the disparate frequencies as described by Praba, the helices of the two interleaved arrays are oppositely wound, so that a right-hand-circularly-polarized antenna element is adjacent a left-hand-circularly-polarized antenna element, which results in some degree of rejection of the cross-polarization signal from the adjacent elements, and thereby tends to reduce mutual coupling between the antenna elements of the two interleaved arrays.
A disadvantage of such axial-mode helical antennas is that they are three-dimensional, rather than two-dimensional as are the slot antenna elements. Thus, an array of axial-mode helical antenna elements occupies a volume defined by the area of the array and the axial length of the longest helical element. A further disadvantage of such helical antenna elements is the need for feeding each helical antenna element at a location adjacent the ground plane. Various schemes have been applied to couple signal between a transmission line and the proximal (ground-plane) end of the antenna element. One simple scheme couples signal to (or from) the helical antenna element by means of a coaxial transmission line, in which the center conductor is contiguous with (connected to or coupled to) the helical element and the outer conductor is connected to the ground plane. "Wye" coupling, which is coupling of the center conductor of a feed coaxial transmission line to a location on the helical element which is spaced away from the ground plane, has been used to improve the bandwidth of the antenna-element-plus-feed. When waveguide feed is desired, a probe must be coupled into the waveguide to intercept the electromagnetic field, and connected to the proximal end of the conductor of the axial-mode helical antenna element. A major problem with such probes is that of matching the impedance of the helical-element-plus-probe to the impedance of the waveguide. Impedance match is measured in various ways, one of which is the "return loss." A good (low) return loss over a range of frequencies ordinarily translates into an antenna with a broad operating frequency range.
Bifilar and multifilar helical antenna elements are also known, and have advantageous characteristics, such as operation in the absence of a ground plane. However, the feeding of bifilar antennas in most situations requires the use of one or more balanced-to-unbalanced (balun) converters, and such baluns may be difficult to make at some of the higher frequencies at which operation is desired. Further, even if a balun is available, some types of multifilar antennas may disadvantageously require that the feed of the helically disposed conductors of the antenna element be at the distal end of the helical structure, remote from a ground plane or support structure.
Improved array antenna elements are desired.